The present invention relates to a heat exchanger for a refrigerated merchandiser, and more particularly, the present invention relates to a heat exchanger having a heat exchanger coil for transferring heat between a refrigerant in the heat exchanger coil and air flowing over the heat exchanger coil.
In conventional practice, supermarkets and convenience stores are equipped with refrigerated merchandisers, which may be open or provided with doors, for presenting fresh food or beverages to customers while maintaining the fresh food and beverages in a refrigerated environment or product display area. Typically, cold, moisture-bearing air is provided to the product display area of the merchandiser by passing an airflow over the heat exchange surface of an evaporator. A suitable refrigerant is passed through the evaporator, and as the refrigerant evaporates while passing through the evaporator, heat is absorbed from the air passing through the evaporator. As a result, the temperature of the air passing through the evaporator is lowered for introduction into the product display area. The refrigerant is then directed from the evaporator to a condenser, which transfers heat from the refrigerant to the environment.
Some conventional heat exchangers include round-tube plate-fin coil assemblies, which typically have relatively poor efficiency. Over time, dirt and debris accumulates on these conventional heat exchangers, particularly in stand-alone merchandiser applications located in areas near high customer traffic volume, which can further decrease the heat exchanging efficiency of the associated coil assembly. The fouling caused by dirt, debris, and oils causes an increase in undesirable air-side pressure drop, which lowers the volume of air flowing through the condenser coil. The lower volume of air through the condenser coil reduces the amount of heat rejection from the condenser coil and impedes refrigeration performance by increasing the compressor refrigerant pressure, leading to overall system inefficiency and possible compressor failure. Generally, the greater the tube and fin densities that exist in conventional evaporators and condensers leads to more efficient performance of the associated coil with regard to heat transfer between the refrigerant and surrounding air. However, relatively large tube and fin densities make these heat exchangers more susceptible to fouling by accumulation of foreign matter on the coils.
Other conventional heat exchangers include bare tube coil assemblies to avoid excessive build-up of foreign matter on the coils. However, these bare-tube heat exchangers typically have relatively poor and undesirable heat transfer efficiency due to a relatively small heat transference area. Typically, air flowing over the bare tube forms a thin slow moving fluid layer (i.e., a boundary layer) having decreased pressure in flow direction. Often, substantial wake formation occurs on the trailing side of the bare tube and the airflow moves away from bare tubes that are downstream from the leading bare tube, which undesirably affects heat exchanger performance.
Generally, the performance of heat exchangers deteriorates as foreign matter builds up on the heat exchanger coil and the free flow of air through the heat exchanger becomes restricted, and in extreme cases halted. The build up of foreign matter on the heat exchanger coils reduces the amount of air that can pass between the coils, which restricts the heat exchange capability of the heat exchanger. Flow of adequately refrigerated air to the product display area decreases as a consequence of foreign matter buildup, which necessitates relatively frequent cleaning of the heat exchanger coils that may be detrimental to the food and/or beverage products, since the products may be allowed to warm-up to a temperature above desired temperature ranges. Cleaning conventional heat exchangers also typically results in increased energy expenditures and increased costs due to the relatively high frequency of the cleaning operation and a relatively large amount of energy that is required to initially “pull down” the air temperature in the product display area to an acceptable temperature after a cleaning operation.